Volumetric Analysis

• Volumetric analysis

  Volumetric analysis uses a standard solution to determine the concentration of another solution
• What is the most common type of volumetric analysis?

  A titration
• How are standard solutions prepared?

  Accurate dilution of an existing solution or from a primary standard
• Preparation from a primary standard (solid)

  Accurately weigh out required mass of primary standard

  Dissolve in a small volume of solvent (usually water) in a beaker

  Transfer solution and rinsings to volumetric flask

  Make up to graduation mark with solvent

  Stopper and invert flask to mix
• Preparation by dilution of a standard solution

  Pipette required volume of standard solution into volumetric flask

  Make up to graduation mark with deionised water

  Stopper and invert to mix
• Required characteristics of primary standards

  High purity (impurities would alter concentration)

  Stable as a solid in air and when in solution (so can be stored without change to composition)

  Soluble in water (so a solution can be made)

  Reasonably high formula mass (reduces errors when weighing)
• Examples of common primary standards:

  Sodium carbonate, EDTA, silver nitrate and oxalic acid
• Why is sodium hydroxide not a primary standard?

  It absorbs moisture from the air and is unstable as both a solid (absorbs moisture) and in solution (reacts with carbon dioxide) and it has a relatively low GFM which means the sodium hydroxide solution must be standardised before being used in volumetric analysis.
• What should you do if you need a primary standard such as sodium hydroxide for a titration?

  A solution of approximate concentration must be prepared and then standardised, this involves carrying out another titration first to determine the accurate concentration of the sodium hydroxide, the standardised solution can be used in the required analysis
• Standard solution

  A solution of accurately known concentration.
• What are the types of titrations from Nat 5/Higher?

  • Acid-base titrations.
  • Redox titrations based on reactions between oxidising and reducing agents.
• Describe a complexometric titration

  These involve the formation of a complex between a metal ion and a complexing agent e.g. EDTA.

  Titrating with EDTA is a useful way of determining the concentration of metal ions in a solution as it reacts with metal ions in a 1:1 ratio.

  Murexide indicator (which usually turns purple at the end point) is usually used in EDTA titrations, however the starting colour / colour change is determined by the specific metal ion.
• Describe the features of a back titration

  • A back titration is a technique used to work out the quantity of a substance in a solid with a low solubility (which means a standard solution cannot be made).
  • It can be carried out if the direct titration is too slow or there is no suitable indicator for it.
• Describe how a back titration is carried out:

  • The substance being analysed (chemical A) is reacted with an excess volume of a reactant of known concentration (chemical B)
  • The resulting mixture is titrated with a chemical that reacts with chemical B (chemical C).
  • This allows us to calculate the number of moles of chemical B in excess and therefore the number of moles of chemical A which reacted with it.